Peripheral vascular endothelial function declines progressively with advancing age in humans, increasing the risk for atherosclerotic and ischemic vascular disease. In addition to its role in maintaining vascular health, the endothelium plays an important role in the regulation of local vascular tone. Recent evidence indicates that the red blood cell (RBC) can act as a "sensor" and releases ATP during mismatches in oxygen demand and delivery, and this ATP can evoke vasodilation and improve local blood flow under such conditions via binding to purinergic (P2y) receptors on the endothelium. Our preliminary data indicates that aging is associated with BB impaired forearm vascular control during specific physiological stressors in which ATP-mediated vasodilation has been documented to be involved. Thus, the overall goal of this research program is to directly test the UU hypothesis that endothelium-dependent ATP-mediated vasodilation is impaired in aging humans, and that this is related to impaired vascular responses during specific physiological stressors. To test our hypothesis we will address the following specific aims: (1) we will determine whether the forearm vasodilator responses to local intra-arterial administration of ATP is impaired in older compared with young healthy adults; and (2) we will determine whether RBC release of ATP during rhythmic handgrip exercise, systemic hypoxia, and combined exercise and systemic hypoxia is reduced with age and relates to impaired forearm vasodilation in older adults. The methods employed to address these aims are state-of-the-art and involve local (intra-arterial) administration of various study drugs at rest, and measurements of forearm venous plasma ATP concentrations in young and older healthy humans during physiological stressors. The findings from the proposed studies should provide unique insight into whether (a) endothelium-dependent ATP-mediated vasodilator responsiveness is reduced with age, (b) whether RBC release of ATP is reduced with age, and (c) whether impairments in both the vascular responsiveness to, and RBC release of, ATP contribute to reduced vasodilator responses during specific physiological stressors that evoke mismatches in oxygen demand and delivery. Our findings could have significant implications for understanding how endothelial dysfunction relates to impaired local vascular control during physiological (e.g., exercise, hypoxia) and pathophysiological (e.g., coronary and cerebrovascular ischemia) conditions in older healthy and diseased humans. Aging is associated with an increased risk for cardiovascular disease. The studies in this application are designed to understand how impaired blood vessel function might contribute to a reduced ability of older adults to respond to conditions in which not enough blood is being delivered to specific tissues, and could provide ideas on how to eventually improve cardiovascular health of older adults. [unreadable] [unreadable] [unreadable]